1. Field of the Invention
The present invention relates broadly to evaporative emission control systems of automobiles. More specifically, the present invention is concerned with a fuel cut-off valve for preventing discharge of liquid fuel from a fuel tank while admitting release of fuel vapor.
2. Description of the Prior Art
Automobile fuel tank is generally connected through a vapor vent line to a charcoal canister to capture and recover fuel vapor generated in the fuel tank. In some types of fuel system, the vapor vent line also permits the fuel tank to breathe whenever the fuel tank headspace is subjected to a negative pressure. To prevent liquid fuel from leaking through the vapor vent line to the canister during jolting, tilting, cornering or roll-over of the automobile, a fuel cut-off valve is provided at the top of the fuel tank. This valve is also referred-to in the art as a roll-over valve.
A typical design of an earlier fuel cut-off valve is illustrated in FIGS. 9 and 10 of Japanese Patent Kokai Publication No. 2-112658. As shown therein, the earlier fuel cut-off valve has a small vapor vent orifice which is controlled by a valve member formed atop of a float member. When the liquid level in a float chamber is raised due to cornering of vehicle or for any other reasons, the float urges the valve member in contact with the vapor vent orifice to preclude leakage of liquid fuel. As the liquid level is then lowered, the float descends to re-open the valve member whereby communication between the tank headspace and the canister is resumed.
Generally, it has been the conventional practice to design the vapor vent orifice small enough to ensure that the valve member is readily unseated under the action of the gravity acting on the float member to re-open the vent orifice even in the event that a substantial pressure differential exists across the vent orifice due to the positive pressure that would prevail in the fuel tank headspace.
Recently, onboard refueling vapor recovery systems have been proposed to totally suppress or substantially reduce the emission of fuel vapor during refueling. In these systems, the fuel filler tube of the fuel tank and the fuel dispensing nozzle are sealed with each other to ensure that all fuel vapor in the fuel tank is forwarded to the canister throughout the refueling operation.
During refueling, fuel vapor present or generated in the tank is displaced as liquid fuel is dispensed into the tank. The flow rate of fuel vapor as displaced out of the fuel tank is equal to or even larger than the flow rate of the fuel dispenser which may attain, for example, 15-40 litters per minute. In order to smoothly discharge fuel vapor to the canister during refueling, it is desirable that the diameter of the vapor vent orifice of the fuel cut-off valve be made as large as possible. However, as the vapor vent orifice is enlarged, the effective cross-sectional area of the valve member of the fuel cut-off valve is correspondingly increased so that the valve member is subjected to a substantial pressure force if a pressure differential exists across the vapor vent orifice. Accordingly, once the valve member has closed, it tends to remain closed when it should be re-opened. This condition is referred-to in the art as "sticking" or "corking".
Various valve designs have been proposed to avoid corking. For example, Japanese Patent Kokai Publication No. 2-112658 cited above and Szlaga U.S. Pat. No. 5,028,244 propose use of a movable upper closure member which is separate from the float member and which is adapted to open and close the large-diameter vapor vent orifice. The upper or main closure member is provided therethrough with a second vapor vent orifice of a smaller diameter. The small-diameter second orifice is adapted to be controlled by a second or lower closure member integral with the float member.
The upper or main closure member is guided through a plurality of arms by the float member which, in turn, is guided by the valve body. The main closure member normally rests by gravity upon the float member. As the float is lifted, the main closure member is brought into engagement with the large-diameter main vapor vent orifice.
When re-opened, the lower second closure member is first disengaged from the main upper closure member which, in turn, is disengaged from the main valve seat. The main closure member is designed to be pulled down by the float member through a lost motion mechanism. To this end, each of the arms is provided at the lower end with a hook adapted to engage a flange or shoulder of the float member, the arrangement being such that the upper closure member is pulled down by the float to re-open the small-diameter orifice only when the float member has stroked down for a certain distance.
The advantage of this design is that the diameter of the main vapor vent orifice can be made larger so as to increase the flow rate of fuel vapor displaced from the tank to the canister during refueling.
However, the problem involved in this design is that, due to the presence of the lost motion mechanism, the responsiveness of the upper or main closure member is unsatisfactory to the extent that it is not re-opened until the float member has fully stroked. Under certain operating conditions wherein, for example, the vehicle repeats cornering or continues to run over a rough terrain so that the float member remains situated somewhat halfway of its stroke, the main closure member tends to stay closed. This would result in an excessive pressure rise in the fuel tank if the amount of fuel vapor generated due to agitation of liquid fuel or due to temperature exceeds the amount of fuel vapor released through the small-diameter second orifice.
Accordingly, an object of the present invention is to provide a fuel cut-off valve having an improved responsiveness.
Another object of the present invention is to provide a fuel cut-off valve which is capable of effectively releasing fuel vapor even under a condition wherein fuel vapor is vigorously generated.
Another problem associated with the conventional fuel cut-off valves cited above is that the axial guide for the main closure member is not positive. Because the main closure member simply rests by gravity upon the float member, it tends to bounce on the second or lower closure member when the vehicle vibrates. The main closure member is guided through a certain radial clearance by the float member which, in turn, is guided by the valve body through another additional radial clearance. Accordingly, the radial clearances to which the main closure member is subjected are two fold. As a consequence, there is a risk that the main closure member is tilted or offset from the main valve seat in response to vibration thereby resulting in an imperfect closure.
Accordingly, a still another object of the invention is to provide a fuel cut-off valve which has an improved sealing or shut-off capability.